ABSTRACT
Field flatness of bevelled intraoperative electron therapy cones were evaluated from dose profiles, taken at dmax, both in the longitudinal plane (long axis direction) and the transversal plane (short axis direction), and were found to depend strongly on the setting of the x-ray collimators. Dose gradients in the longitudinal plane of 10-12% were found for collimator settings of 5 mm larger than the cone diameter for low energies, while the dose gradient were smaller for higher energies, both decreasing with larger collimator setting. The dose increase, relative to the central dose, of the hot spots observed in the profile of the transversal plane were in the range of 5-10% for high energies and large collimator settings, decreasing to less than 3% for low electron beam energies and smaller collimator setting. A decrease in virtual focus to surface distance (VFSD) was found to accompany the increasing dose gradient in the longitudinal plane with decreasing collimator setting, this due to increased scatter of electrons at collimator level. Increasing scatter with smaller collimator setting is also indicated by the increase in photon contamination ranging from 1-2.5% for low energies and 3.5-5% for high energies.
